Indium plating



Patented Feb. 21 1950 INDIUM PLATING Ealing, London, and John DavidFrederick Green,

Ernest Salmon,

London, England, assignors to Vandervell Products Limited, London,England N Drawing.

Application July 11, 1945, Serial No.

604,524. In Great Britain May 22, 1943 Section 1, Public Law 690, August8, 1946 Patent expires May 22, 1963 6 Claims; ((1204-45) This inventionrelates to indium plating and particularly in its relation to bearingsurfaces.

Indium has been electrodeposited in various ways and has become ofimportance in recent years when the plating has been applied on top of abearing material such as lead, lead-copper, lead-bronze and cadmiumalloys, etc. to fortify the bearing metal against corrosion due to theacids formed by oxidation of the lubricating oil.

The main difiiculty experienced in practice is the provision of a stableplating bath.

Attempts have been made to stabilise an indium cyanide bath by additionsof saccharidespreferably a high grade dextrose, and while these attemptshave been partially successful there has always been the disadvantagethat the bathshad to be made up extremely carefully and aged for some 48hours during which time the solution darkened and a dark brown sludgeprecipitated which contained indium. The plating rate oi these knownsolutions started at about 15 mg./amp'./min. (milligrams, per ampere,per minute) and fell ofi rapidly to about 5-8 mg./amp./min. Morerecently attempts have been made to develop a successful acid bath basedon the employment of indium sulphate. A bath of this character, however,also suffers from the serious disadvantage that it isdiflicult todissolve the indium metal in sulphuric acid except by prolongedrefluxing and the chemical control required is very delicate,particularly in respect of its pH value. The solution not being alkalineit has no cleaning properties unlike the older type of bath. Moreover,the cathode efiiciency is only about 50%. The acidity is a disadvantagealso from the point of view of corrosion of th parts of the articles notrequired to be plated. A further disadvantage is that unless adequateprecautions are taken to agitate the bath thoroughly, the plate isstratified.

We have by research discovered a new type of indium cyanide dextroseplating bath which overcomes the disadvantages inherent in the processesin prior use and the primary characteristic of our invention is the usein the bath of additions of alkali metal hydroxides such, for example,of sodium or potassium hydroxide.

The success of our new bath depends, however, partly on the procedurefor making up the solution, employing the before-mentioned hydroxideadditions made to stabilise the bath. By these means it is possible toprepare a stable concentrate suitable for renewing a depleted bath.

Thus our invention provides means whereby it is possible to preparestable concentrates and baths of the cyanide, alkali hydroxide typewhich operate. at very high cathode eificiencies approaching 100% i. e.having a plating rate of some 20-23 mg./amp./min.

The following procedure will explain by way of example the method ofmanufacture of a stable indium-potassium cyanide-potassium hydroxideconcentrate. For the best results the use of pure chemicals isrecommended.

According to this procedure g. indium metal are dissolved in an excessof concentrated hydrochloric acid and the solution is evaporated untilcrystallisation of indium chloride begins. It is preferred that theindium chloride shall not be allowed to evaporate to dryness before itis used in the next stage of the process. The indium chloride which hascrystallised is then dissolved in distilled water and the volume made upto about 100 ccs.

A second solution is then prepared by dissolving 110-150 gms. ofpotassium cyanide in 200 to 250 ccs. of cold distilled water and to thisis added a cold solution of 30 g. dextros in 50 ccs. distilled water. Athird solution isprepared by dissolving 50-100 gms. of potassiumhydroxide in 50-100 ccs. distilled water and cooling thoroughly.

article would be serious it is The first solution is then added to thesecond drop by drop (forinstance from a burette) at the rate ofabout 10ccs. per minute with constant stirring, the temperature being maintainedpreferably below 30 C. and in any case below 10 C.

When all of the first solution has been added to the second the thirdsolution can be added in the normal way. The resulting concentrate willbe of clear golden yellow colour and will contain approximately gms. perlitre indium, 60 gms. per litre dextrose, 200-300 gms. per litre potassium cyanide, 100-200 gms. per litre potassium hydroxide.

This concentrate can be used as a plating solu tion asit is, but as dragout losses 1. e. losses consequent on adhesion of the liquid to theplated preferable to dilute to at least 1 litre when the plating ratewill still be high (about cathode efficiency) and drag out lossesconsiderably less important. At some further sacrifice in speed ofplating the solution may be still further diluted up to a maximum ofsome five or six litres when plating will be slow but losses of indiummetal due to dragout will be very much less important.

In fact the plating rate and drag out can be balanced at will to suitthe particular work in hand.

These baths have 'the'great advantage that they are completely stableand at the same time are usable over a very wide range of composition,voltage, current density and temperature and can be adjusted to operateat rates of deposition approaching 100% cathode efficiency.

The sodium and potassium baths are distinguished from the earliercyanide dextrose baths which have been suggested by additions of thecorresponding alkali metal hydroxide and it appears likely that for asodium bath containing say 30 g./1. indium, 25 g./l. sodium hydroxidewill be sufficient though we prefer 30 g./l. .minimum. In the case ofpotassium 35 g./l. potassium hydroxide appears to be suficient thoughweprefer 4o g./l. minimum. If solutions are operated with lowerconcentrations of indium lower concentrations of alkali metal hydroxidewill he satisiactory. It is to be understood that the scope of ourinvention is not to be limited by these minimum figures the dominantfeature of the invention being the principle of stabilising the bath byadditions of alkali metal hydroxide in quantities suihcient to result ina stable bath.

In practice we prefer not to operate the baths at concentrationsso'close to the minimum and it will be found that the baths will operatebetween the following Wide limits:

The ratio of dextrose to indium is preferably at least 1:1 but shouldnot be below 1:4 and in any case dextrose should be at least 5 g./l.

Potassium Potassium cyanide -500 Potassium hydroxide 8-450 Sodium Sodiumcyanide 8-200 Sodium hydroxide 6-200 Additions of sodium and potassiumcyanide and sodium and potassium hydroxide can be 'increased to thelimit of their solubility so far as we have at present ascertained,though there is little advantage in so doing and at extremely highconcentrations of alkali hydroxide some deposition of indium hydroxideappears to take place on the anode, while the current is passing. Thecathode current density may 'be up to at least 20 amperes per squaredecimetre. The temperature may be from room temperature or even lower upto the boiling point. Anodes may be of an inert conducting material suchas mild steel, carbon or platinum though it-ispossible with solutions ofcertain compositions within the given range to operate with a solubleindium anode particularly at elevated-temperatures. It is to beunderstood that though the concentrations of each constituent may varythroughout the given range, the concentrations of the severalconstituents are not independent of each other and must be varied in duerelation.

The deposits produced are smooth white and adherent. The baths have theadvantage that if for any reason they are seriously contaminated or themetal is required it can be recovered by heating th solutions andplating out.

It is to be understood that the details may be considerably variedwithout departing from the principles enumerated above and suitablesaccharides other than dextrose may be used.

Examples of suitable stable baths prepared according to the principlesof our invention:

80 g./1. ini- Sodium Indium, 30 g./l.

Dextrose, 30 g./l.

Total NaCN (sodium cyanide), g./l. (initially) Free NaCN, 20 g./l.(maintained) Total NaOI-I (sodium hydroxide), 80 g./l. (initially) FreeNaOH, 40 -g./l. (maintained) Room temperature (20 C.)

Current density, 2 amperes per square decimetre Inert anode Rate ofdeposition, 19 mg./amp./min.

Cathode efiiciency, 78%

The same solution at 80 C. at the same current density gave anefiiciency of 95%.

Since these baths are largely used for plating thin flash coatings forsubsequent diffusion into a basis metal or .alloy the use of the highercurrent densities referred to will be advantageous.

What we claim and desire to secure by Letters Patent is:

1. An aqueous indium cyanide dextrose electrolytic bath forelectrodepositing indium consisting of from 5 to 40 grams per liter ofindium,

5 to 40 grams per liter of dextrose with a ratio by Weight of dextroseto indium of 1 to 1, a cyanide of the group consisting of sodium cyanideand potassium cyanide of from 1 to 5 times the weight of indium presentand the corresponding hydroxide of the above cyanide in quantities offrom 6 to grams per liter.

2. .An aqueous indium cyanide dextrose electrolytic bath forelectrodepositing indium consisting of .from .5 to 40 grams per liter ofindium, 5 to 4.0 grams per liter of dextrose with a ratio by weight ofdextrose to indium of 1 to 1, sodium cyanide in amounts 1 to 5 times theweight of indium present and 6 to 135 grams per liter of sodiumhydroxide.

3. An aqueous indium cyanide dextrose electrolytic bath forelectrodepositing indium consisting of from 5 to 40 grams per liter ofdextrose with a ratio by weight of dextrose to indium of l. to 1,potassium cyanide in amounts 1 to 5 times the weight of indiumpresentand 6 to 135 grams .per liter of potassium hydroxide.

4. A method of preparing a stable aqueous in- .dium cyanide dextroseelectrolytic bath which consists of adding indium trichloride in smallquantities to .a solution composed of dextrose and a cyanide of thegroup consisting of potassium cyanide and sodium cyanide, cooling andmixing the solution by rapid agitation during the addition to keep thetemperature below 30 to 40 C. and subsequently rapidly adding thehydroxide of the metal whose cyanide was included in the first solution.

5. A method of preparing a stable aqueous indium cyanide dextroseelectrolytic bath which consists of adding indium trichloride in smallquantities to a solution composed of dextrose and potassium cyanide,cooling and mixing the solution by rapid agitation during the additionto keep the temperature below 30 to 40 C. and subsequently rapidlyadding potassium hydroxide.

6. A method of preparing a stable aqueous in- DAVID FREDERICK GREEN.JOHN ERNEST SALMON.

:0 Metal Finishing,

REFERENCES CITET) The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Transactions of the AmericanElectrochemical Society, volume 65, pages 377-380 (1934).

Iron Age, Dec. 19, 1940, pages 35-3S.

July 1944, pages 405-407. Chemical Age, Oct. 6, 1945, pages 314 and 315.

1. AN AQUEOUS INDIUM CVANIDE DEXTROSE ELECTROLYTIC BATH FORELECTRODEPOSITING INDIUM CONSISTING OF FROM 5 TO 40 GRAMS PER LITER OFINDIUM, BY WEIGHT OF DEXTROSE TO INDIUM OF 1 TO 1, A CYANIDE OF THEGROUP CONSISTING OF SODIUM CYANIDE AND POTASSIUM CYANIDE OF FROM 1 TO 5TIMES THE WEIGHT OF INDIUM PRESENT AND THE CORRESPONDING HYDROXIDE OFTHE ABOVE CYANIDE IN QUANTITIES OF FROM 6 TO 135 GRAMS PER LITER.